The control of content is important in order to protect programming from, for example, nonpaying customers. A conventional communications system, such as a cable television system, therefore, typically applies an encryption scheme to digital television content in order to prevent unrestricted access. Once a system operator chooses an encryption scheme, the operator installs all of the necessary headend equipment (e.g., Scientific-Atlanta's conditional access software and associated equipment). The receiving devices (e.g., set-tops) located at the subscriber's premises must be compatible with the encryption scheme in order to decrypt the content for viewing. Due to the (at least partial) proprietary nature of conditional access systems, however, an operator is prevented from installing different set-tops that do not have the proper decryption keys and decryption algorithms. If the operator wishes to install different set-tops that decrypt a different conditional access system, the operator would also have to install a second proprietary encryption system to overlay the incumbent encryption system in order to use both set-tops.
It would be to the operator's advantage to be able to select set-tops from any manufacturer and easily implement different encryption/decryption schemes in the system without totally duplicating the headend equipment and utilizing substantially extra bandwidth. For example, a portion, but not all, of the data required for full presentation of a television program is encrypted according to one encryption scheme and then the same portion of data is encrypted again according to a second encryption scheme. The first encryption scheme corresponds to the legacy or incumbent set-top and the second encryption scheme corresponds to the non-legacy or overlay set-top. The remaining data is transmitted unduplicated in the clear to minimize the bandwidth impact.
Unique integer values commonly referred to as packet IDs (PIDs) are used to associate packets carrying elementary streams of a program in a single or multiple program transport stream. Known implementations of partial dual encryption involve duplicating only certain packets in a transport stream tagged with a certain PID. An additional or secondary PID is then mapped to each duplicated component to distinguish between duplicated content. Various known methods such as time slicing, MTH & N packet encryption, data structure encryption, or system information (SI) encryption are used to select the portions of the information as critical packets to be encrypted. Critical packets are packets selected for encryption based upon their importance to the proper decoding of the program content. For example, in MPEG content streams, critical packets are preferably packets containing higher-level headers such as picture headers, GOP headers, etc. Also, various encryption methods such as those found in PowerKEY®, from Scientific-Atlanta, Inc., may be utilized to encrypt the portions once selected while leaving other portions in the clear. However, original PIDs, commonly referred to as legacy or primary PIDs, continue to tag the packets encrypted with the legacy encryption as well as the other packets sent in the clear. By using primary and secondary PIDs, the decoder located in a set-top box can determine which packets are to be decrypted using the encryption method associated with that particular set-top box. In other words, regardless of the manner in which packets are selected for encryption and the encryption used, PID mapping or manipulation techniques are used to distinguish between multiple encryptions. For example, the legacy set-top decrypts the packets tagged with the primary PIDs and the overlay set-top decrypts the packets tagged with the secondary PIDs. The legacy set-top ignores the encrypted packets with the secondary PIDs and the overlay set-top ignores the encrypted packets with the primary PIDs. Set-tops, whether legacy or overlay, can determine which portions of the transport stream are transmitted and received in the clear. Once identified, the packets transmitted in the clear pass through the descramblers unaffected.
In particular, prior-art set-top boxes send each stream through a PID remapper, a PID filter, and then to a video decoder. The present invention, however, eliminates the need for a PID remapper. The overlay set-top box uses a dual filter system (FIG. 4) for each video or audio stream and then sends the output to the video or audio decoder. Advantageously, this is an improvement that reduces the cost and the complexity of the conditional access overlay system.